Piping systems in nuclear power plants are often designed for pressure and mechanical loadings (including seismic loads) and operating thermal transients. In the last few decades a number of failures have occurred due to thermal stratification caused by the mixing of hot and cold fluids under certain low flow conditions. Such stratified temperature fluid profiles give rise to circumferential metal temperature gradients through the pipe leading to high stresses causing fatigue damage. In this work, thermal stresses due to such temperature gradients have been calculated using a finite element method. The peak stresses calculated by this method has been used for fatigue evaluation. In addition the stresses due to thermal striping associated with stratification have also been independently assessed for high cycle fatigue. The method outlined in this paper is a simplified conservative procedure to obtain stratification stresses.

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